Basic Genetics and the Australian Shepherd Part Two: Multiple Alleles First printed in the Aussie Times by George P. Johnson The first article in this series dealt with the inheritance of base coat color and pattern in Aussies. In review, these two traits are each controlled by a single gene and there are two genetic variants possible per gene; black and red for base color and merle and solid for pattern. Each variation of a gene is known as an allele and an individual inherits two alleles for each gene. One allele is inherited from the sire and the other is inherited from the dam; these alleles may be the same (homozygous) or different (heterozygous). A more complicated situation occurs when there are multiple (more than two) alleles for a gene. In these cases, there are multiple genotypes (genetic makeups) and phenotypes (appearances) that are possible, not just the three genotypes/two phenotypes for color and three genotypes/three phenotypes for pattern. Nevertheless, these situations are analyzed with a Punnett Square in the same way as traits for which there are only two alleles (see Figures 1 and 2 on pages 76 & 77 in the September-October 1996 Aussie Times [Click here for the article]). While blood type in humans (ABO system, with alleles A, B and O) is probably the most familiar multiple allele system, copper and white trim in Australian Shepherds are also examples of multiple allele genes for which there are known three and four alleles, respectively. There is some debate as to the number of alleles for each of these genes in dogs as well as differences in their notations; therefore, the number of alleles reported here for copper and white trim in Aussies and their notations have been standardized with the work of C. A. Sharp (personal communication). Extensive descriptions of the genetics of color are provided in the works by Clarence Little (The Inheritance of Coat Color in Dogs) and Malcom Willis (Genetics of the Dog). Inheritance of Copper Trim in the Australian Shepherd Copper trim in Aussies is governed by the A locus (a gene or a position on a chromosome) which controls pigmentation at the "tan points areas." This is most easily seen in black dogs when the base pigment is modified into copper on the muzzle, at the eyebrows, inside the ears, on the legs and under the tail. There are three alleles known in the Aussie for this locus: Ay (sable); at (copper at the tan points areas), and a (self or full pigmentation, no copper). The alleles form a series with Ay dominating at dominating a. This gives six possible genotypes and three possible phenotypes with each dog having two alleles at this locus. Because Ay dominates the other two alleles, Parent 1 Tan Trim (ata) (self carrier) at (tan trim allele) a (self allele) Parent 2 Tan Trim (ata) (self carrier) at (Tan trim) atat (Tan Trim) ata (Tan Trim) (self carrier) a (self allele) ata (Tan trim) (self carrier) aa (self color) Figure 1. Heterozygous Tan Trim (ata) Mated to a Heterozygous Tan Trim (ata) a dog only has to have a single copy of the Ay allele to be sable. Therefore, a dog with the sable phenotype could have any of the following genotypes at the A locus: AyAy, Ayat, or A y a . Sable is not an acceptable color under the ASCA or AKC breed standards, and the appearance of sable differs depending on the breed. Often, dogs carrying the sable allele are reddish or yellowish in appearance (depending on other genes) and there may be individual hairs with alternating bands of darker and lighter color along their length. Although a sable dog phenotypically is a non-standard color, genotypically it may carry the tan points allele (Ayat) and pass it to its offspring producing a dog that is breed-standard. Sable was once seen occasionally in the breed, but since it is dominant and has been selected out for about 30 years, it is now extremely rare or absent in the gene pool. A dog with tan points is acceptable under the ASCA breed standard and may be a carrier of self color (a) allele. If two tan-pointed dogs that each carry the recessive allele for self color are mated,there is a one-in-four chance that they will produce an offspring with self color - no copper trim (see Figure 1). The dogs are called bicolor if they have white markings and are without copper trim. Self color (a) is recessive to the sable alleles and tan trim alleles, a self color dog is known to be homozygous for this allele (aa). Therefore, mating two dogs that have no copper trim will result only in puppies that have no copper trim, with one exception that we will discuss below. If self color offspring result from mating a dog with tan trim, that dog had to be heterozygous and was a carrier of the self color allele. The exception noted above is the K gene. This gene is epistatic to A, meaning it can counteract the action of A. There are two alleles, K, which will not allow the expression of tan pigment due to A locus alleles anywhere on the dog, and k, which has no effect on tan pigment. Therefore, if a dog has at least one copy of the dominant K it will not be sable nor will it have tan trim. A dog that is self colored due to K would have no tan trim but might have tan trim offspring if it was heterozygous (Kk). However, if the dog was also aa it would have no tan trim. Inheritance of White Trim in the Australian Shepherd White trim in Aussies is controlled by the S locus which produces white pigmentation on the forehead, muzzle, throat, neck, chest, belly, legs, feet and at the tip of the tail. This locus is extremely complex because four alleles are involved, alleles interact with each other, and there are apparently alleles at another gene or genes which modify or influence the alleles at this locus. These modifiers increase or decrease the extent of white trim and can cause overlaps between the various phenotypes at this locus; the result is a transition from no white to an extremely white dog with no clear break-points between phenotypes. The four alleles at this locus are: S (self, little or no white); si (Irish spotting pattern); sp (piebald or pattern white); and, s (extreme white). Unlike the alleles for copper trim, these alleles do not show simple dominance over one another. A dog that is homozygous for the self allele (SS) has no white trim or may have just a spot of white on the chest and toes. The variation is apparently due to modifying alleles at another locus that restrict pigment production in these areas. A fully pigmented dog (black or red) is acceptable under the ASCA breed standard and would be considered to be a bicolor if copper trim is present in the tan points areas. The Irish spotting allele (si) Parent 1 Irish Spotted (Ssi) S (Self allele) si (spotted allele) Parent 2 Irish Spotted (Ssi) S (Self) SS (Self, no white) Ssi (irish spotted) si (spotted) Ssi (irish spotted) sisi (irish spotted) Figure 2. Mating of Two Heterozygous Irish Spotted (Ssi) Dogs is responsible for producing white trim in the familiar areas; it may do so when homozygous (sisi) or when paired with the self allele (Ssi); if two dogs are heterozygous for Irish spotting are mated they may produce a fully pigmented or self dog (Figure 2) as well as dogs with Irish spotting. To complicate matters, Irish spotting may also be due to the pairing of the piebald and extreme white alleles with the self allele (Ssp, Ss), although a dog with this genotype may have more extensive white areas than a dog with the genotype Ssi or sisi. In a pattern white or piebald individual, white extends over large areas of the body in an irregular pattern (white body splashes). Three genotypes may be responsible for the piebald or pattern white appearance depending on whether the individual is homozygous for the piebald allele (spsp) or is heterozygous with the Irish spotting (sisp) or the extreme white (sps) alleles. Because of possible confusion between a piebald individual and a homozygous merle (usually excessively white), piebald is unacceptable within the ASCA breed standard. An extreme white Aussie (ss) is nearly white to all white in color. These individuals are also unacceptable within the ASCA breed standard (as with piebald) due to confusion with homozygous merles but should not be confused with albinos (cc) controlled by the C locus. Coat Color, Pattern and Trim Variations As far as is known, the genes controlling the tan points areas (A locus), base color (B locus), pattern (M locus) and white trim (S locus) are on different chromosomes (not linked) and do not influence one another. Therefore, any and all combinations are possible among the genes and alleles at these loci. A Punnett square constructed to account for all possible combinations of these 4 genes and their alleles (excluding the non-standard sable, piebald and excessive white alleles) would have 256 cells (64 times larger than a Punnett Square for color or pattern alone, 16 times larger than for color and pattern considered together, and 4 times larger than for color, pattern and either copper or white trim). This means that there are 256 possible combinations of color, pattern, white trim and copper trim in an Australian Shepherd. Given this type of possible variation, is it any wonder that our dogs are each unique and individual in appearance? Other Multiple Allele Systems Coat color, pattern, ticking and trim colors have been used as examples in this and the previous article because they are individually simple genetically (controlled by one gene), are easily seen, and are relatively easy to understand. As the number of genes under consideration increases along with the number of alleles per gene, the combinations possible dramatically increase. The focus of the next article in this series will be on DNA "fingerprinting." While this is a complex and complicated topic, it is based on an understanding of the inheritance of "alleles" at loci, just as we have seen with the alleles for color, pattern, ticking and copper and white trim. Questions from the Membership One of the purposes of this series of articles is to answer questions from ASCA's membership. If you have a question about something in one of these articles or anything related to the genetics of the Australian Shepherd, contact me at the address below, by phone, or by email. If I don't know the answer I will search until I do. Your question will be answered privately or in an upcoming issue of the Aussie Times. George P. Johnson Department of Biological Sciences Arkansas Tech University Russellville, AR 72801 Phone: 501-968-0312 FAX: 501-964-0837 Email: george.johnson@mail.atu.edu